1. Field of the Invention
The invention relates to systems for feeding and transporting documents and to document hoppers used in these systems.
2. Background Art
A typical system for feeding and transporting documents includes a feeder in the document feeding portion of the system, and a series of roller pairs or belts in the document transporting portion of the system. In the feeding portion of the system, the feeder acts to separate and feed documents singly, in order, from a stack. In the transporting portion of the system, the roller pairs and/or belts convey the documents, one at a time, past other processing devices such as readers, printers, and sorters that perform operations on the documents. The feeder is typically a feed wheel, but may take other forms. Further, the components in the transporting portion of the system may take a variety of forms. An existing document feeder is shown in U.S. Pat. No. 6,199,854. That patent describes a document feeder with a variable speed separator.
In existing systems for feeding and transporting documents, operations that depend on the position of the document are generally performed in the transport stage, or transporting portion of the system. For example, U.S. Pat. No. 5,848,784 describes a document separation apparatus. That patent describes the downstream acceleration/deceleration of documents with pinch rollers to adjust document spacing. U.S. Pat. Nos. 5,419,546; 5,437,375; 5,439,506; 5,509,648; 5,671,919; and 5,908,191 describe examples of other document operations.
As modern document handling devices are typically fitted with an automatic feeder mechanism to singly introduce documents into a track for further processing, a hopper is usually associated with the feeder so that the machine can load a number of documents to be processed. As feed rates increase, and feed mechanism reliability improves, there are advantages to making hopper capacity larger.
The difficulty with making hopper capacity larger is one of consistency. Document feeders need to have a supply of documents presented to the feeding mechanism in a consistent manner. This is a task of the document hopper. The variety of documents used in different applications make such consistent presentation difficult.
There is an ideal set of forces for feeding a document in a given feeder. The closer each document can be to this ideal set of forces, the better feeder performance will be. More specifically, the feeder must apply enough pinch force to allow the document to feed, but not so much as to result in the tearing apart of the document during feeding. As hopper capacity is increased, the variation in force against the stack between that needed to move a full hopper of documents and that needed to move the last few documents and provide an acceptable force to the document being fed is increased.
Typically, some form of mechanical intervention urges the document stack along in the hopper but the mechanical intervention may not compensate as the document stack diminishes. For a large capacity hopper, it is possible that the mechanical intervention may result in correct pinch force when the hopper is full, but too much pinch force when the hopper nears empty, or that the mechanical intervention may result in correct pinch force when the hopper is near empty, but too little pinch force when the hopper is full.
An existing form of mechanical intervention used to urge the document stack along in the hopper toward the feeding mechanism applies a generally constant force to the document stack with a flag. This form of mechanical intervention may limit the hopper capacity because the applied force to the stack must result in acceptable forces on a feeding document when moving a full hopper of documents and when moving the last few documents. Various approaches have been taken for driving the flag to produce the flag force or flag weight against the document stack.
Additional background information may be found in U.S. Pat. Nos. 6,474,637; 6,417,221; and 6,260,841.
The horizontal force with which the documents are presented to the feeder, F, is a function of the flag force, Ff, the weight of the document stack, W, and the friction coefficient of the document against the hopper floor, μ:F=Ff−W·μ
The weight of the document stack is a summation of the weight of each document in the stack. As the hopper gets larger, the document stack can get larger. As the document stack gets larger, the weight difference between a stack of large documents and a stack of small documents grows. For proper feeder operation, the horizontal force with which documents are presented to the feeder, F, needs to fall within a range. The range for each feeder is determined by several factors including geometry and friction element effectiveness. For a given feeder, the range is essentially fixed. So the large hopper needs to be designed to carefully control the horizontal presentation force, F, despite increasing variations due to large hopper size.
Although various approaches have been taken to controlling the flag force, Ff, there has been no practical way to know the weight of the documents that had been loaded. Flag force can be tailored to some middle ground, but it cannot react to each document. As a result, it becomes difficult to keep the horizontal presentation force, F, within the optimal range.
Controlling the friction coefficient of documents against the hopper floor is one approach to controlling the variation in horizontal presentation force. However, this approach does have shortcomings. For example, very low friction materials can be delicate, low friction properties tend to degrade with wear, documents are loaded into the hopper with some violence, friction coefficient varies with humidity, and friction coefficient versus paper is largely unknown. Therefore, controlling variation with an approach that attempts to directly maintain a very low friction coefficient is not always reliable.
For the foregoing reasons, there is a need for an improved system for feeding and transporting documents that urges the document stack along in the hopper in a way that provides a consistent presentation of documents to the feeder.